1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a tubular thermoplastic resin film having a rough inner surface.
2. Description of Related Art
Thermoplastic resins, such as flexible vinyl chloride resin, are excellent in processability, physiological stability, transparency, and cost, and are therefore extensively used as a constituent of medical goods having an annular cross-section, such as blood bags, transfusion bags, and dialysing fluid bags.
Such medical goods having an annular cross-section are conventionally formed by the tubular film process which is the extrusion process. In this tubular film process, molten thermoplastic resin is supplied into an annular resin passage formed between a mandrel and a die ring which surrounds the mandrel. The thermoplastic resin is discharged in an annular fashion from a discharge port in a state in which the temperature of the portion of the mandrel located close to the discharge port is set to a value lower than the temperature of the die ring while the air is blown into the interior of the resin to form a tubular film before the temperature of the resin does not become lower than the softening point. When the tubular film has been inflated to a desired cross-sectional dimension, the film is rapidly cooled to a temperature lower than the softening point and is thereby hardened. The hardened film is folded into two portions by pinch rolls and then wound up by a take-off roll.
The sheet formed by the blown film extrusion has specular inner and outer surfaces. Therefore, blocking phenomenon (undesired mutual adhesion of the material which occurs at a temperature lower than the melting point of that material) often occurs on the medical goods made of the thermoplastic resin film manufactured by the tubular film process. When this phenomenon happens, the medical goods cannot be opened readily. This is inconvenient for charging of medicine or blood into the medical goods.
Accordingly, the applicants of this invention have proposed in Japanese Patent Laid-Open No. Sho 62-19431 the method of preventing blocking which occurs on the inner surfaces of the thermoplastic resin film. In this conventional method, the molten thermoplastic resin is discharged in an annular fashion from the discharge port of the resin passage formed between the mandrel and the die ring in a state where the temperature of the portion of the mandrel is set to a value lower than the temperature of the die ring. According to this conventional method, the molten thermoplastic resin is not substantially cooled within the resin passage. However, only the portion of the thermoplastic resin which corresponds to the inner surface of the annular resin film is rapidly cooled immediately before discharge from the discharge port, causing rapid surface shrinkage to occur on the inner surface of the film which is discharged. This results in formation of the finely irregular inner surface of the film. Blocking does not occur on such a film when the inner surfaces thereof are brought into contact with each other.
However, in the annular thermoplastic resin film manufactured by the above-described conventional technique, the overall inner surface thereof has fine irregularities. This means that the opposing two inner surfaces of the tubular film have fine irregularities and are thus rough. This makes provision of sufficient transparency on the tubular film difficult. When such a tubular film is used for medical goods, the content of the medical goods cannot readily be recognized visually. Thus, handling of the medical goods deteriorates.